Let's say you knew a guy, a really bad man, who beat his kids, very likely even killed one, who you were told on very good authority was going to cause you and your neighbors great harm. While he'd been restricted to house arrest, had been forced to wear one of those ankle tracker things, he'd also been researching ways to unlock it, and looking into ways to get some new guns.
The police don't listen to you - they figure the guy isn't going anywhere, he's under too close a watch - but you have to protect your family, so you go and kill the guy.
However, you then learn that while maybe he'd hoped someday to hurt you, that the people who told you he was going to kill you were all wrong, they'd misheard things.
So... what does this make you? A bad man, probably better off dead, but does this justify your actions?